1. Field of the Invention
The invention relates to a method for controlling engine air/fuel ratio.
2. Background Art
After-treatment devices such as Lean NOx Traps (LNT) are being devised to adsorb NOx emissions from diesel engines in an effort to meet future emission standards. LNTs can be very effective in trapping NOx but need to be periodically cleaned through a regeneration process. A rich environment (close to an air/fuel ratio of 14) is required to regenerate the trap. While fuel injection in the combustion chamber and/or fuel injection in the exhaust stream can effectively reduce the exhaust air/fuel ratio, they both present some limitations in terms of how much fuel can be injected. Too much fuel injected in the combustion chamber after the main combustion event (late post injection) can result in wall wetting and engine damage or affect the engine torque curve when injected too early. Injecting large quantities of fuel in the exhaust stream could require large and bulky injectors.
For the foregoing reasons, there is a need for an improved method for controlling engine air/fuel ratio.